Catalyzed polyol gel breaker compositions

ABSTRACT

It has been discovered that fracturing fluid breaker mechanisms are improved by the inclusion of a catalyzed polyol alone that directly degrades the polysaccharide backbone, and optionally additionally by removing the crosslinking ion, if present. That is, viscosity reduction (breaking) occurs by breaking down the chemical bonds within the backbone directly. The gel does not have to be crosslinked for the method of the invention to be successful, although it may be crosslinked. In one non-limiting embodiment, the polyol has at least two hydroxyl groups on adjacent carbon atoms. In another embodiment, the polyols are simple sugars and sugar alcohols, and may include mannitol, sorbitol, glucose, fructose, galactose, mannose, lactose, maltose, allose, etc. and mixtures thereof. The catalyzing metal ion may employ a metal selected from Groups VIB, VIIB, VIII, IB, and IIB of the Periodic Table (previous IUPAC American Group notation).

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part application of U.S.patent application Ser. No. 09/900,343 filed Jul. 3, 2001, now allowed.

FIELD OF THE INVENTION

[0002] The present invention relates to gelled treatment fluids usedduring hydrocarbon recovery operations, and more particularly relates,in one embodiment, to methods of “breaking” or reducing the viscosity oftreatment fluids containing gelling agents used during hydrocarbonrecovery operations.

BACKGROUND OF THE INVENTION

[0003] Hydraulic fracturing is a method of using pump rate and hydraulicpressure to fracture or crack a subterranean formation. Once the crackor cracks are made, high permeability proppant, relative to theformation permeability, is pumped into the fracture to prop open thecrack. When the applied pump rates and pressures are reduced or removedfrom the formation, the crack or fracture cannot close or healcompletely because the high permeability proppant keeps the crack open.The propped crack or fracture provides a high permeability pathconnecting the producing wellbore to a larger formation area to enhancethe production of hydrocarbons.

[0004] The development of suitable fracturing fluids is a complex artbecause the fluids must simultaneously meet a number of conditions. Forexample, they must be stable at high temperatures and/or high pump ratesand shear rates that can cause the fluids to degrade and prematurelysettle out the proppant before the fracturing operation is complete.Various fluids have been developed, but most commercially usedfracturing fluids are aqueous based liquids that have either been gelledor foamed. When the fluids are gelled, typically a polymeric gellingagent, such as a solvatable polysaccharide is used. The thickened orgelled fluid helps keep the proppants within the fluid. Gelling can beaccomplished or improved by the use of crosslinking agents orcrosslinkers that promote crosslinking of the polymers together, therebyincreasing the viscosity of the fluid.

[0005] The recovery of fracturing fluids may be accomplished by reducingthe viscosity of the fluid to a low value so that it may flow naturallyfrom the formation under the influence of formation fluids. Crosslinkedgels generally require viscosity breakers to be injected to reduce theviscosity or “break” the gel. Enzymes, oxidizers, and acids are knownpolymer viscosity breakers. Enzymes are effective within a pH range,typically a 2.0 to 10.0 range, with increasing activity as the pH islowered towards neutral from a pH of 10.0. Most conventional boratecrosslinked fracturing fluids and breakers are designed from a fixedhigh crosslinked fluid pH value at ambient temperature and/or reservoirtemperature. Optimizing the pH for a borate crosslinked gel is importantto achieve proper crosslink stability and controlled enzyme breakeractivity. One disadvantage of enzyme breakers is that they tend to berelatively expensive. Oxidizer breakers are relatively less expensive,but can be dangerous if not handled properly, and further have atechnical gap of not being useful between about 160 to about 230° F.(about 71 to about 110° C.).

[0006] It would be desirable if a viscosity breaking system could bedevised to break fracturing fluids gelled with borate crosslinkedpolymers by directly breaking down the polysaccharide backbone, whetheror not the backbone is crosslinked.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to providea method for breaking the viscosity of polymer-gelled aqueous treatmentfluids used in hydrocarbon recovery operations.

[0008] It is another object of the present invention to provide acomposition and method for breaking polysaccharide gelled aqueous fluidsby breaking down the polysaccharide backbone directly.

[0009] Still another object of the invention is to provide a method andcomposition for breaking the viscosity of aqueous fluids gelled withpolymers that can provide better clean up of the crosslinked polymer.

[0010] In carrying out these and other objects of the invention, thereis provided, in one embodiment of the invention a method for breakingviscosity of aqueous fluids gelled with polysaccharides that involvesadding to an aqueous fluid gelled with at least one polysaccharide, atleast one low molecular weight polyol. A metal ion is added to theaqueous fluid in an amount effective to catalyze the polyol to breakdown the polymer backbone directly. The metal ion may be employed on acatalyst substrate. The metal ion is selected from the Periodic TableGroups VIB, VIIB, VIIIB, IB and IIB. The sequence of addition of thepolyol and the metal ion is not critical and they may be added together.

[0011] In other non-limiting embodiments of the invention, the methodsand compositions for breaking the viscosity of aqueous fluids gelledwith polysaccharides do not require lowering the pH of the fluid, andfurther do not require removing or sequestering any crosslinking ions,particularly any borate or other ions. However, crosslinking ions may bepresent, and the methods of the invention may coincidentally removecrosslinking ions from crosslinked polysaccharides. In anothernon-limiting embodiment of the invention, the methods and compositionsdo not require the use of an enzyme or an oxidizing breaker.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a graph of the effects of Fe⁺² and Cu⁺² at 200° F. (93°C.) in 30 pptg (3.6 kg/m³) borate crosslinked guar, showing littlebreaking effect;

[0013]FIG. 2 is a graph of the effects of Mn⁺², Co⁺² and Zn⁺² at 200° F.(93° C.) in 30 pptg (3.6 kg/m³) borate crosslinked guar, showing littlebreaking effect;

[0014]FIG. 3 is a graph of the results of 0.5 pptg (0.06 kg/m³) glucosepolyol catalyzed by Fe⁺² and Cu⁺² at various concentrations at 200° F.(93° C.) in 30 pptg borate crosslinked guar, showing pronounced breakingeffect;

[0015]FIG. 4 is a graph of the results of 0.5 pptg (0.06 kg/m³) glucosepolyol catalyzed by Mn⁺² and Co⁺² at various concentrations at 200° F.(93° C.) in 30 pptg borate crosslinked guar, showing pronounced breakingeffect;

[0016]FIG. 5 is a graph of the results of 0.5 pptg (0.06 kg/m³) glucosepolyol catalyzed by Mo⁺² and Zn⁺² at various concentrations at 200° F.(93° C.) in 30 pptg borate crosslinked guar, showing pronounced breakingeffect;

[0017]FIG. 6 is a graph of the results of 0.5 pptg (0.06 kg/m³)alpha-lactose polyol catalyzed by Fe⁺² and Co⁺² at variousconcentrations at 200° F. (93° C.) in 30 pptg borate crosslinked guar,showing pronounced breaking effect;

[0018]FIG. 7 is a graph of the results of 1.0 pptg (0.12 kg/m³)alpha-lactose polyol catalyzed by Fe⁺² and Mo⁺² at variousconcentrations at 200° F. (93° C.) in 30 pptg borate crosslinkedhydroxypropyl guar, showing pronounced breaking effect; and

[0019]FIG. 8 is a graph of the results of 2.0 pptg (0.24 kg/m³) alkylglucoside polyol catalyzed by Fe⁺², Cu⁺² and Mn⁺² at variousconcentrations at 200° F. (93° C.) in 30 pptg borate crosslinked guar,showing pronounced breaking effect.

DETAILED DESCRIPTION OF THE INVENTION

[0020] A unique fracturing fluid breaker mechanism in which thefracturing fluid's viscosity is reduced (or is “broken”) by use ofcatalyzed polyols has been discovered. It is not necessary, and in somecases not desirable, for oxidizer or enzyme breakers to also be presentin the methods of this invention. In the context of this invention,polymer degradation is dependent primarily on the type and amount ofpolyol, the type and amount of metal ion catalyst (and whether or notthe catalyst is complexed or chelated), and fluid temperature. Otherphysical and chemical conditions also have a role in the breakingactivity of the polyols on polysaccharide gelled fluids, includingwhether the polymer fluid is crosslinked (has a three dimensionalstructure), fluid pH, and whether there are present glycols, alcohols,amino acids, salinity, and the like, and in what proportion these othercomponents are present and mixtures thereof.

[0021] The polyols of this invention are defined in one non-limitingembodiment as polyols having at least one hydroxyl group on two adjacentcarbon atoms. The adjacent carbon atoms may have more than one hydroxylgroup, and the polyol may have more that two adjacent carbon atoms, eachhaving at least one hydroxyl group. In another embodiment of theinvention, the polyols are simple sugars (e.g. mono and disaccharidessuch as glucose, fructose, lactose, maltose, etc.), sugar alcohols,(e.g. glycerol, sorbitol, xylitol, and mannitol), oligosaccharides,derivatives of sugar (e.g. alkyl glucosides, alkyl sorbitans,glucosamine, gluconate, etc.) and the like. In another embodiment of theinvention, the polyols may have one of the following formulae:

[0022] where n is from 2 to 5, and the hydroxyls may be in the cis ortrans orientation. In another embodiment of the invention, the polyolsare acids, acid salts, fatty acids (alkyl glycosides), and alcohol,alkyl and amine derivatives (glycosylamines) of monosaccharides andoligosaccharides. Specific examples of polyols falling within thesedefinitions include, but are not necessarily limited to, mannitol (mannasugar, mannite), sorbitol (D-sorbite, hexahydric alcohol), xylitol,glycerol, glucose, (dextrose, grape sugar, corn sugar), fructose (fruitsugar, levulose), maltose, lactose, tagatose, psicose, galactose, xylose(wood sugar), allose (β-D-allopyranose), ribose, arabinose, rhamnose,mannose, altrose, ribopyranose, arabinopyranose, glucopyranose,gulopyranose, galatopyranose, psicopyranose, allofuranose, gulofuranose,galatofuranose, glucosamine, chondrosamine, galactosamine, ethyl-hexoglucoside, methyl-hexo glucoside, aldaric acid, sodium aldarate,glucaric acid, sodium glucarate, gluconic acid, sodium gluconate,glucoheptonic acid, sodium glucoheptonate, and mixtures thereof. In onenon-limiting embodiment of the invention, the molecular weight of thesimple polyols may range from about 65 to about 500, where an alternateembodiment for the molecular weight ranges from about 90 to about 350.Useful oligosaccharides may have molecular weights ranging from about450 to about 5000 in one non-limiting embodiment, with most ranging fromabout 480 to about 1000 in another non-limiting embodiment.

[0023] It will be appreciated that derivatives of these relativelysimple polyols will also find use in the inventive methods andcompositions. Suitable derivatives include, but are not necessarilylimited to, acid, acid salt, alcohol, alkyl, and amine derivatives ofthese saccharides, and mixtures of polyols and/or the derivativesthereof. Specific examples of suitable derivatives include, but are notnecessarily limited to, alkyl glucosides, alkyl polyglucosides, alkylglucosamides, alkyl glucosamines, alkyl sorbitans, alkyl sorbitols,alkyl glucopyranosides, alkyl maltosides, alkyl glycerols and mixturesthereof. The alkyl groups of these derivatives may be C2 to C36straight, branched, or cyclic alkyls.

[0024] The metal ion catalysts of this invention may employ metals fromGroups VIB, VIIB, VIII, IB, and IIB of the Periodic Table (previousIUPAC American Group notation) in one non-limiting embodiment of theinvention. In another non-limiting embodiment of the invention metal ofthe metal ion may be molybdenum, manganese, iron, cobalt, copper, zinc,chromium, nickel, palladium, and combinations thereof. The metal ionsmay be introduced as a part of other compounds, including, but notnecessarily limited to, organometallic complexes with iron, copper,cobalt, manganese, etc.; metals in oxide, sulfate, carbonate, orphosphate compound form as finely ground particles; metal ions platedonto particles, such as proppants; metal ions dispersed within matrix orplated onto surface of zeolites, such as iron, cobalt or copper loadedZSM-5; or metal ions attached to the surface of clays, such as smectite.The metal ions may also be part of an inorganic compound, such as in theforms described above or other compounds. The metal ions may also, inone non-limiting embodiment, be in an encapsulated or pelletized form asa method of delayed release of metal ions. Of course, in onenon-limiting embodiment of the invention, the metal ions may beprecomplexed or chelated with known chelates including, but notnecessarily limited to, gluconate, glucoheptonate, organic acidsincluding citric acid and the like and aminocarboxylic acids includingethylenediamineteteracetic acid (EDTA), nitrilotriacetic acid (NTA) andthe like.

[0025] Only trace amounts of the metal ion catalysts are required.Typical metal ion concentrations may range from about 0.01 to about100.0 ppm, and in another non-limiting embodiment from about 0.1 toabout 10.0 ppm.

[0026] The use of simple sugars, acid sugars, acid sugar salts, alcoholsugars, alkyl glycosides, and glycosylamines to complex or chelateborate ions of the gelled polysaccharides lowers the pH of the boratecrosslinked fluid (if the fluid is borate-crosslinked) and thus itsviscosity.

[0027] Further, the amounts of polyols used in the method of thisinvention are lower than those used in the prior art where some of suchpolyols are used to delay gelling in the first place. In non-limitingembodiments, the amounts of polyols used in the methods of thisinvention may be up to one-twentieth ({fraction (1/20)}) as low as whathas been previously used. In the catalyzed polyol invention herein, theamounts of polyols used are even lower. Further, the use of thecatalyzed polyols of this invention permit the breaking of gels over awider temperature range than is possible with some prior art methods.The catalyzed polyols of this invention may be used from about 80° F.(27° C.) up to about 260° F. (127° C.), in another non-limitingembodiment of this invention the polyols may be used in the range offrom about 160 to about 230° F. (about 71 to about 110° C.).

[0028] Once the pH starts to be lowered through the prescribed mechanismof liberating the borate ions from the gel by the polyols, breaking(viscosity reduction) occurs by uncrosslinking of the fracturing fluid,and by liberating the crosslinking ion, e.g. borate as well, if present.In general, the lower that the pH shifts through the use of a borate ionsequestering product, the more effective and complete the above-listedbreaking mechanisms can be. In other words, because more than onemechanism is used in some cases, a more complete break may be obtained.Complete borate uncrosslinking and up to 80 to near-100 percent backbonereduction (polysaccharide chain degradation) can be achieved with theselection and proper use of a catalyzed polyol breaker.

[0029] It will be appreciated that breaking of the gel by reducing thepH of the fluid and removing at least a portion of the borate ion (ifpresent) from the crosslinked polymer and attacking the polysaccharidebackbone directly do not happen instantaneously or when the metal ionand the polyol are added to the fluid, nor should it. Rather, thesemechanisms act over time or eventually. This time delay is necessary tocomplete the fracturing portion of the operation and the optionalsetting of the proppant. The time delay will also vary depending on theparticular requirements of each individual fracturing job and cannot bespecified in advance.

[0030] A value of the invention is that a fracturing fluid can bedesigned to have enhanced breaking characteristics. Importantly, betterclean-up of the crosslinked polymer from the fracture and wellbore canbe achieved thereby. Better clean-up of the polymer directly influencesthe success of the fracture treatment, which is an enhancement of thewell's hydrocarbon productivity.

[0031] Most conventional borate crosslinked fracturing fluids andbreakers are designed from a fixed crosslinked fluid pH value at ambientand/or reservoir temperature. By having products that can lower the pHof the fracturing fluid at reservior temperature, such as the materialsof the invention, the breaking of the fluid can be enhanced beyondexisting conventional materials or methods for fracturing. The result ismore enhanced breaking of the fracturing fluid over conventionalmaterials and methods, which gives better clean-up of the crosslinkedpolymer from the fracture and wellbore.

[0032] One advantage of the catalyzed polyol breakers of this inventionis that they have little, if any toxicity or environmental concerns, andthus, are safer to ship, handle and use as compared with somealternative breakers. The polyol breakers of this invention have anotheradvantage of being relatively less expensive than conventional enzymebreakers. Oxidizer breakers are also relatively inexpensive, but theyhave a technical gap of not being useful from about 160 to about 230° F.(about 71 to about 110° C.).

[0033] In order to practice the method of the invention, an aqueousfracturing fluid is first prepared by blending a hydratable polymer intoan aqueous fluid. The aqueous fluid could be, for example, water, brine,aqueous based foams or water-alcohol mixtures. Any suitable mixingapparatus may be used for this procedure. In the case of batch mixing,the hydratable polymer and the aqueous fluid are blended for a period oftime sufficient to form a hydrated solution. The hydratable polymer thatis useful in the present invention can be any of the hydratablepolysaccharides having galactose or mannose monomer units and arefamiliar to those in the well service industry. These polysaccharidesare capable of gelling in the presence of a crosslinking agent to form agelled base fluid. For instance, suitable hydratable polysaccharides arethe galactomannan gums, guars and derivatized guars. Specific examplesare guar gum and guar gum derivatives. The preferred gelling agents areguar gum, hydroxypropyl guar and carboxymethyl hydroxypropyl guar. Themost preferred hydratable polymers for the present invention are guargum and carboxymethyl hydroxypropyl guar and hydroxypropyl guar.

[0034] The amount of polysaccharide included in the fracturing fluid isnot particularly critical so long as the viscosity of the fluid issufficiently high to keep the proppant particles suspended thereinduring the fluid injecting step. Thus, depending on the application, thehydratable polymer is added to the aqueous fluid in concentrationsranging from about 15 to 60 pounds per thousand gallons (pptg) by volumeof the total aqueous fluid (1.8 to 7.2 kg/m³). The most preferred rangefor the present invention is about 20 to about 40 pptg (2.4 to 4.8kg/m³).

[0035] In addition to the hydratable polymer, the fracturing fluids ofthe invention may optionally include a crosslinking agent, such as aborate crosslinking agent. The crosslinking agent can be any of theconventionally used borate crosslinking agents that are known to thoseskilled in the art. This includes any of the boron salts or boric acidas borate crosslinking agents. Guar and derivatized guar gels, which arecrosslinked by the addition of borate ion donating materials, arepreferred within this embodiment over other crosslinking agents becausethey clean up faster and yield higher sand pack permeability than guargels crosslinked with other crosslinking agents. However, othercrosslinking agents can be used with this embodiment besides borate,which may include, but are not limited to, titanates, zirconates, andother metallic and semi-metallic crosslinkers.

[0036] In the case of borate crosslinkers, the crosslinking agent is anymaterial that supplies borate ions into solution. The amount of borateions in solution is dependent on pH. Thus, the crosslinking agent can beany convenient source of borate ions, for instance the alkali metal andthe alkaline earth metal borates and boric acid. A preferredcrosslinking additive is preferably a common type of borax present inthe range from about 0.25 to in excess of 10.0 pptg of the total aqueousfluid (0.03 to in excess of 1.2 kg/m³). Preferably, the concentration ofcrosslinking agent is in the range from about 1.0 to about 3.0 pptg(0.12 to 0.34 kg/m³) by volume of the total aqueous fluid.

[0037] Propping agents are typically added to the base fluid just priorto the addition of the crosslinking agent. Propping agents include, butare not limited to, for instance, quartz sand grains, glass and ceramicbeads, bauxite grains, walnut shell fragments, aluminum pellets, nylonpellets, and the like. The propping agents are normally used inconcentrations between about 1 to 14 pounds per gallon (120-1700 kg/m³)of fracturing fluid composition, but higher or lower concentrations canbe used as the fracture design requires. The base fluid can also containother conventional additives common to the well service industry such assurfactants, biocides, non-emulsifiers and the like.

[0038] In one non-limiting embodiment of the invention, the suitablepolyol materials for use in the invention include those described above,such as monosaccharides, oligosaccharides, and their acid, acid salt,alcohol, alkyl, and amine derivatives, in one non-limiting embodiment ofthe invention. In a different preferred embodiment, polyols of formulae(I), (II), and (III), are preferred in another non-limiting embodimentof the invention.

[0039] Any or all of the above polyol materials may be provided in anextended release form such as encapsulation by polymer or otherwise,pelletization with binder compounds, absorbed on a porous substrate, anda combination thereof. Specifically, the materials may be encapsulatedto permit slow or timed release of the polyol materials. In non-limitingexamples, the coating material may slowly dissolve or be removed by anyconventional mechanism, or the coating could have very small holes orperforations therein for the material within to diffuse through slowly.For instance, polymer encapsulation coatings such as used in fertilizertechnology available from Scotts Company, specifically POLY-S® productcoating technology, or polymer encapsulation coating technology fromFritz Industries could possibly be adapted to the methods of thisinvention.

[0040] It is difficult, if not impossible, to specify with accuracy theamount of the polyol that should be added to a particular aqueous fluidgelled with borate crosslinked polymers to fully break the gel, ingeneral. For instance, a number of factors affect this proportion,including but not necessarily limited to, the particular polymer used togel the fluid; the particular polyol used to break the gel; thetemperature of the fluid; the starting pH of the fluid; the nature andthe concentration of the pH buffers; and the complex interaction ofthese various factors. Nevertheless, in order to give an approximatefeel for the proportions of the polyol to be used in the method of theinvention, the amount of material added may range from about 0.1 toabout 30.0 pptg (about 0.012 to about 3.6 kg/m³), based on the totalweight of the fluid; preferably from about 0.5 to about 30.0 pptg (about0.06 to about 3.4 kg/m³); most preferably from about 1.0 to about 20.0pptg (about 0.12 to about 2.4 kg/m³). However, in the catalyzed polyolembodiment of the invention, the amount of polyol can be lowered evenfurther, to between about 0.1 to about 10 pptg (about 0.012 to about 1.2kg/m³) based on the total volume of fluid, and between about 0.5 toabout 10 pptg (about 0.06 to about 1.2 kg/m³) in an alternate embodimentof the invention.

[0041] It will be appreciated that in some embodiments of the invention,the amount of polyol necessary to break a particularpolysaccharide-gelled aqueous fluid will depend upon the particularpolyol used.

[0042] It is necessary, in some embodiments, to add pH buffers to thegelled aqueous fluid to increase the pH to generate active borate ionfor crosslinking the polymers. Suitable buffers include, but are notnecessarily limited to sodium hydroxide, potassium hydroxide, sodiumcarbonate, sodium sesquicarbonate, potassium carbonate, sodiumbicarbonate, and mixtures thereof. The amount of the pH buffer may rangefrom about 0.5 to about 30.0 pptg (about 0.06 to about 3.6 kg/m³), basedon the total volume of the entire fluid, preferably from about 1 toabout 20 pptg (about 0.12 to about 2.4 kg/m³).

[0043] In a typical fracturing operation, the fracturing fluid of theinvention is pumped at a rate sufficient to initiate and propagate afracture in the formation and to place propping agents into thefracture. A typical fracturing treatment would be conducted by hydratinga 20 lb to 30 lb/1000 gal water (weight/volume) (about 2.4 to about 3.6kg/m³) glactomannan-based polymer, such as guar, in a 2% (w/v) (166lb/1000 gal (19.9 kg/m³)) KCl solution at a pH ranging from about 6.0 toabout 8.0. For crosslinking this pH range may be from about 8.8 to about10.5. The polyol is added at this stage. It should be understoodthroughout the specification and claims that more than one polyol may beemployed at a time. During the actual pumping, as described, the pH ofthe ambient temperature guar gel is raised by the addition of a bufferto about 9.5 to about 12.5, followed by the addition of the crosslinkingagent, proppant, and other additives, if required.

[0044] The present invention will be explained in further detail in thefollowing non-limiting Examples that are only designed to additionallyillustrate the invention but not narrow the scope thereof.

General Procedure for Examples

[0045] Using a Waring blender, 4.8 mls of Drilling Specialties SlurryGuar (guar gum slurried in a glycol ether based fluid suspension) washydrated for 15 minutes within 500 mls of distilled water containing 10grams KCl salt. This first fluid mix was then poured into a 500 ml widemouth Nalgene plastic bottle and labeled fluid No.1. A second fluid wasmixed for 15 minutes as listed above and then 1.0 ppm Fe⁺² was added(0.5 mls from a solution of 0.227% bw FeCl₂ in Dl water) and mixed foran additional 1 minute on the Waring blender. The second fluid was thenpoured into a 500 ml wide mouth Nalgene plastic bottle and labeled No.2.A third fluid was mixed for 15 minutes as listed above, and then 0.045grams of alpha D-glucose (0.75 pptg glucose) was added to the hydrateguar fluid and mixed an additional 1 minute on the Waring blender. Thethird fluid was then poured into a 500 ml wide mouth Nalgene plasticbottle and labeled fluid No.3. A fourth fluid was mixed for 15 minutesas listed above, and 0.045 grams of alpha D-glucose (0.75 pptg glucose)along with 1.0 ppm Fe⁺² was added (0.5 mls from a solution of 0.227% bwFeCl₂ in Dl water) and mixed for an additional 1 minute on the Waringblender. The fourth fluid was then poured into a 500 ml wide-mouthNalgene plastic bottle and labeled No.4. Then 1.0 mls of a 47% bw/bwK₂CO₃ solution was added to each of the fluid samples, and each samplewas capped and shaken vigorously for 60 seconds. Next 1.2 mls of boratecrosslinker (from Benchmark Research) was added to fluid No. 1 and wasthen quickly capped and shaken vigorously for 60 seconds. 1.2 mls ofborate crosslinker was then added to fluid No.2 and was then quicklycapped and shaken vigorously for 60 seconds. 1.2 mls of boratecrosslinker was then added to fluid No.3 and was then quickly capped andshaken vigorously for 60 seconds. 1.2 mls of borate crosslinker was thenadded to fluid No.4 and was then quickly capped and shaken vigorouslyfor 60 seconds. The samples were immediately placed in a pre-heatedwater bath at 200° F. (93° C.) and visually observed every 15 minutesfor viscosity reduction difference between the samples over a two hourperiod.

[0046] Not all components were added in order to determine the breakingeffects with just certain components alone. As will be seen, sample No.2 with only the metal ions did not give much gel breaking, if any,during the first two hour period in the 200° F. (93° C.) water bath, andis comparable to sample No. 1 which does not have metal ions or polyolpresent. Sample No.3 with polyol (e.g. 0.75 pptg glucose) lost viscositynoticeably faster during the first two hour period in the 200° F. waterbath. However, the use of metal ions together with polyols, fluid sampleNo.4, lost viscosity even faster than when polyols were used alone(fluid sample No.3) indicating a catalytic mechanism, and at least asynergistic effect. Most gel breaking that resulted in fluid sample No.4occurred over the first hour.

FIGS. 1-2

[0047]FIGS. 1 and 2 demonstrate the effects of using only the metalcations to break 30 pptg (3.6 kg/m³) borate crosslinked guar, withoutusing any polyol, at various cation concentrations. FIG. 1 presents theresults for Fe⁺² and Cu⁺², where FIG. 2 presents the results for Mn⁺²,Co⁺², and Zn⁺². As can be seen, while some breaking occurred, it wasgenerally not significant. In the case of 200 ppm Zn⁺², essentially nobreaking occurred.

FIGS. 3-5

[0048]FIGS. 3, 4 and 5 demonstrate the effects of using the same metalcations to break 30 pptg (3.6 kg/m³) borate crosslinked guar, but inconjunction with glucose polyol, at various cation concentrations. Thetopmost curve in each FIG. represents the case where no metal ion isused and while some breaking occurs, it is not very rapid. FIG. 3presents the results for Fe⁺² and Cu⁺², and it may be seen that bothmetal ions are very effective, and that as the amount of metal ionincreases, the gel breaks faster. FIG. 4 presents the results for Mn⁺²and Co⁺², and similar results are seen in that the use of the metal ionsgreatly increased the gel breaking, and the gel breaking acceleratedwith increasing amounts of metal ion. FIG. 5 shows similar results forMo⁺² and Zn^(+2,) although 4.0 ppm Mo⁺²gave a much more dramatic resultthan did 2.0 ppm Mo⁺². The use of 100 Zn⁺² provided greatly improvedbreaking as compared with twice that amount when no glucose polyol wasused in FIG. 2.

FIGS. 6-7

[0049]FIGS. 6 and 7 demonstrate the effects of using the metal cationsFe⁺² . Co⁺² and Mo⁺² to break 30 pptg (3.6 kg/m³) borate crosslinkedguar, but in conjunction with alpha-lactose polyol, at various cationconcentrations. Note that the guar used in FIG. 7 was hydroxypropylguar. The topmost curve in each FIG. represents the case where no metalion is used and while some breaking occurs, it is not very rapid. FIG. 6presents the results for Fe⁺² and Co^(+2,) and again it may be seen thatboth metal ions greatly increased the gel breaking, and that as theamount of metal ion increases, the gel breaks faster. FIG. 7 presentsthe results for Fe⁺² and Mo^(+2,) and similar results are seen in thatthe use of the metal ions was, and the gel breaking accelerated withincreasing amounts of metal ion.

FIG. 8

[0050]FIG. 8 demonstrates the effects of using the metal cations Fe⁺²,Cu⁺² and Mn⁺² to break 30 pptg (3.6 kg/m³) borate crosslinked guar, butin conjunction with alkyl glucoside polyol, at various cationconcentrations. Again, the topmost curve represents the case where nometal ion is used and while some breaking occurs, it is not very rapid.It may again be noticed that the use of metal ions catalyzes thebreaking reaction, and that increasing amounts of metal ion acceleratesthe breaking. Divalent iron ion consistently provides some of the mostrapid gel breaking over all of the Figures.

[0051] Besides metal ions other chemical components may influence thecatalyzed polyol activity. Components such as ethylene glycol andethanol, if present in the fluid may interact negatively with thecatalyzed polyol breaking process. At low concentration, glycols oralcohols will slow down the catalyzed polyol breaking activity and athigh concentrations they can significantly decrease the polyol breakingactivity (polymer degradation over time). Oxidizers, such as sodiumpersulfate and sodium bromate, may also slow the catalyzed polyolbreaking activity with borate crosslinked guar.

[0052] It is additionally expected that the presence of amine compoundsmay influence the catalyzed polyol breaking activity with boratecrosslinked guar fluids. Specific to amine species and concentrationused, some amines slow the polyol activity and some enhance the polyolbreaking activity. Amines such as sodium iminodisuccinate andpentasodium diethylenetriaminepentaacetate may slow the polyol activity,and amines such as glycine and lysine may enhance the polyol breakingactivity with borate crosslinked guar. If the metal ions are chelatedwith an amino carboxylic acid, or their salts, then only trace amount ofamino carboxylic acid is suggested (such as less than 200 ppm) in orderto not noticeably influence or negate the metal ion catalyzing activitywith the polyol.

[0053] Among other items, the type and amount of pH buffer may also playan important role on the polyol activity. It appears that sodiumhydroxide, and other hydroxide pH buffers, may reduce the catalyzedpolyol breaking activity with polysaccharide gelled fluids. Polyolbreaking activity may be enhanced by the co-use of alkali metalhydroxides together with carbonate pH buffers. Use of carbonate pHbuffers, such as potassium carbonate, sodium sesquicarbonate, sodiumbicarbonate, and other carbonate pH buffers may enhance or increase thepolyol breaking activity with polysaccharides. Other compounds than whathave been listed, which can control fluid pH or chemically alter thecatalyzed polyol activity of breaking a polysaccharide polymer,especially at fluid temperatures of >120° F. (49° C.), may be present inthe fluid and have utility to optimize breaking activity.

[0054] The catalyzed polyol breaker technology of this inventionprovides a number of advantages. The use of the metal ion catalystswidens the temperature application of polyol breaker technology. Thistechnology lowers the cost of breaking gels, providing savings inhydrocarbon recovery operations using fracturing fluids. Further, theuse of polyols in general and iron and manganese in trace amounts ofless than 5.0 ppm provides an environmentally friendly breakertechnology. A particular, non-limiting embodiment of an environmentallyfriendly system would be about 2 to about 5 ppm iron with 5.0 pptg (0.6kg/m³) glucose at 150° F. (66° C.). Additionally, the use of catalyzedpolyol breaker technology of this invention is operationally friendlyand safe to use.

[0055] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof, and has beendemonstrated as effective in providing a method and composition forbreaking polymer gelled fracturing fluids with catalyzed polyols thatbreakdown the polysaccharide backbone directly. However, it will beevident that various modifications and changes can be made theretowithout departing from the broader spirit or scope of the invention asset forth in the appended claims. Accordingly, the specification is tobe regarded in an illustrative rather than a restrictive sense. Forexample, specific combinations or amounts of polymers, optionalcrosslinkers, buffers, polyols, metal ion catalysts, methods ofemploying metal catalysts, and other components falling within theclaimed parameters, but not specifically identified or tried in aparticular composition, are anticipated and expected to be within thescope of this invention.

I claim:
 1. A method for breaking viscosity of aqueous fluids gelledwith polysaccharides comprising adding to an aqueous fluid gelled withat least one polysaccharide, at least one low molecular weight polyol;and adding to the aqueous fluid in any sequence an effective amount of ametal ion to catalyze the polyol to break down the polymer backbonedirectly, where the metal ion is selected from the Periodic Table GroupsVIB, VIIB, VIII, IB and IIB.
 2. The method of claim 1 where in addingthe polyol, the polyol has at least one hydroxyl group on two adjacentcarbon atoms and is selected from the group consisting ofmonosaccharides and disaccharides, and acid, acid salt, alcohol, alkyland amine derivatives of these saccharides.
 3. The method of claim 1conducted in the absence of an oxidizer breaker or an enzyme breaker. 4.The method of claim 1 further comprising raising the pH of the aqueousfluid.
 5. The method of claim 4 where the pH of the aqueous fluid israised with a compound selected from the group consisting of an alkalimetal hydroxide, alkali metal carbonate, bicarbonate, sesquicarbonate,and mixtures thereof.
 6. The method of claim 1 where in adding thepolyol, the polyol is selected from the group consisting of mannitol,sorbitol, xylitol, glycerol, glucose, fructose, maltose, lactose,tagatose, psicose, galactose, xylose, allose, ribose, arabinose,rhamnose, mannose, altrose, ribopyranose, arabinopyranose,glucopyranose, gulopyranose, galatopyranose, psicopyranose,allofuranose, gulofuranose, galatofuranose, glucosamine, chondrosamine,galactosamine, ethyl-hexo glucoside, methyl-hexo glucoside, aldaricacid, sodium aldarate, glucaric acid, sodium glucarate, gluconic acid,sodium gluconate, glucoheptonic acid, sodium glucoheptonate, andmixtures thereof.
 7. The method of claim 1 where in adding the polyol,the amount of polyol added ranges from about 0.1 to about 30.0 pptg(about 0.012 to about 3.6 kg/m³) based on the total volume of fluid. 8.The method of claim 1 further comprising subjecting the polymer and thepolyol to heat, where the temperature ranges from about 80 to about 250°F. (about 27 to about 121° C.).
 9. The method of claim 1 where insubjecting the polymer and the polyol to heat for an effective period oftime, the period of time ranges from about 0.5 to about 48 hours. 10.The method of claim 1 where the metal of the metal ion is selected fromthe group consisting of molybdenum, manganese, iron, cobalt, copper,zinc, chromium, nickel, palladium, and combinations thereof.
 11. Themethod of claim 1 where the metal ion concentration in the aqueous fluidranges from about 0.01 to about 100.0 ppm.
 12. The method of claim 1where the metal ion is complexed or chelated prior to adding.
 13. Themethod of claim 1 where the metal ion is a component of anorganometallic complex.
 14. The method of claim 1 where the metal ion isa component of an inorganic compound.
 15. The method of claim 1 wherethe metal ion is plated onto a particle surface or distributed within asynthetic porous particle.
 16. The method of claim 1 where the metal ionis encapsulated or pelletized.
 17. The method of claim 1 where thepolysaccharide is crosslinked.
 18. The method of claim 17 where thepolysaccharide is crosslinked with an ion selected from the groupconsisting of borate ion, zirconate ion, titanate ion, and combinationsthereof.
 19. A method for breaking viscosity of aqueous fluids gelledwith polysaccharides comprising adding to an aqueous fluid gelled withat least one polysaccharide, at least one low molecular weight polyol,where the polyol has at least one hydroxyl group on two adjacent carbonatoms and is selected from the group consisting of monosaccharides anddisaccharides, and acid, acid salt, alcohol, alkyl and amine derivativesof these saccharides, where the amount of polyol ranges from about 0.1to about 30.0 pptg (about 0.012 to about 3.6 kg/m³) based on the totalvolume of fluid; and adding to the aqueous fluid in any sequence aneffective amount of a metal ion to catalyze the polyol to break down thepolymer backbone directly, where the metal ion is selected from thePeriodic Table Groups VIB, VIIB, VIII, IB and IIB.
 20. The method ofclaim 19 conducted in the absence of an oxidizer breaker or an enzymebreaker.
 21. The method of claim 19 where in adding the polyol, thepolyol is selected from the group consisting of mannitol, sorbitol,xylitol, glycerol, glucose, fructose, maltose, lactose, tagatose,psicose, galactose, xylose, allose, ribose, arabinose, rhamnose,mannose, altrose, ribopyranose, arabinopyranose, glucopyranose,gulopyranose, galatopyranose, psicopyranose, allofuranose, gulofuranose,galatofuranose, glucosamine, chondrosamine, galactosamine, ethyl-hexoglucoside, methyl-hexo glucoside, aldaric acid, sodium aldarate,glucaric acid, sodium glucarate, gluconic acid, sodium gluconate,glucoheptonic acid, sodium glucoheptonate, and mixtures thereof.
 22. Themethod of claim 19 further comprising subjecting the polymer and thepolyol to heat, where the temperature ranges from about 80 to about 250°F. (about 27 to about 121° C.).
 23. The method of claim 19 where insubjecting the polymer and the polyol to heat for an effective period oftime, the period of time ranges from about 0.5 to about 48 hours. 24.The method of claim 19 where the metal of the metal ion is selected fromthe group consisting of molybdenum, manganese, iron, cobalt, copper,zinc, chromium, nickel, palladium, and combinations thereof
 25. Themethod of claim 19 where the metal ion concentration in the aqueousfluid ranges from about 0.01 to about 100.0 ppm.
 26. The method of claim19 where the metal ion is complexed or chelated prior to adding.
 27. Themethod of claim 19 where the metal ion is a component of anorganometallic complex.
 28. The method of claim 19 where the metal ionis a component of an inorganic compound form.
 29. The method of claim 19where the metal ion is plated onto a particle surface, or distributedwithin a synthetic porous particle.
 30. The method or claim 19 where themetal ion is encapsulated or pelletized.
 31. An aqueous fluidcomprising: at least one polysaccharide gel; at least one metal ion inan amount effective to catalyze: at least one polyol, in an amounteffective to eventually reduce the pH of the fluid and break down thepolysaccharide backbone directly; and water.
 32. The fluid of claim 31where the polyol has at least one hydroxyl group on two adjacent carbonatoms and is selected from the group consisting of monosaccharides anddisaccharides, and acid, acid salt, alcohol, alkyl and amine derivativesof these saccharides.
 33. The fluid of claim 31 having an absence of anoxidizer breaker or an enzyme breaker.
 34. The fluid of claim 31 furthercomprising a compound selected from the group consisting of an alkalimetal hydroxide, an alkali metal carbonate, bicarbonate,sesquicarbonate, and mixtures thereof to raise the pH to at least 8.0.35. The fluid of claim 31 where the polyol is selected from the groupconsisting of mannitol, sorbitol, xylitol, glycerol, glucose, fructose,maltose, lactose, tagatose, psicose, galactose, xylose, allose, ribose,arabinose, rhamnose, mannose, altrose, ribopyranose, arabinopyranose,glucopyranose, gulopyranose, galatopyranose, psicopyranose,allofuranose, gulofuranose, galatofuranose, glucosamine, chondrosamine,galactosamine, ethyl-hexo glucoside, methyl-hexo glucoside, aldaricacid, sodium aldarate, glucaric acid, sodium glucarate, gluconic acid,sodium gluconate, glucoheptonic acid, sodium glucoheptonate, andmixtures thereof.
 36. The fluid of claim 31 where the polyol is selectedfrom the group consisting of fructose, glucose, lactose, maltose,sorbitol, or alkyl glucoside.
 37. The fluid of claim 31 where the amountof polyol ranges from about 0.1 to about 30.0 pptg (about 0.012 to about3.6 kg/m³) based on the total volume of fluid.
 38. The fluid of claim 31where the amount of metal ion ranges from about 0.01 to about 100.0 ppmbased on the total volume of fluid.
 39. The fluid of claim 31 where thepolysaccharide is crosslinked.
 40. The fluid of claim 31 where thepolysaccharide is crosslinked with an ion selected from the groupconsisting of borate ion, zirconate ion, titanate ion, and combinationsthereof.